If you were taking medication that was prescribed to you by a doctor, would he want to try it as well?
Perhaps that's a bit of an over exaggeration, But, the specialty products were purchased for you. Your teeth, your skin, your hair, etc.
While typically these types of products can be interchangeable, you've already warned him, that they aren't.
Plus you probably see them as being yours, something that is not cheap / easily available something that you treat as a specialty personal product. You probably don't like him using your stuff because it's yours.
Probably he sees nothing wrong with trying it, dipping his finger into your lotion, trying your fancy shampoo, trying your special toothpaste, popping a couple of your supplements etc.
Was he brought up in a family that was kind of large and everybody just used everybody's stuff? I've known people who were raised with many siblings and most products were just general use for everybody in the household, not for anyone in particular.
Maybe he's just curious, if you say a particular product makes your teeth really white or makes your hair really soft or makes your skin feel really smooth, maybe he just wants to try it. Not all guys are simple in their personal self-care needs, or, if they are, maybe they'd like to be a little bit pampered. Maybe he wants some special products for himself but doesn't know how to ask?